PROJECT SUMMARY Depression is a prevalent problem during pregnancy,1,2 with adverse and potentially enduring correlates and consequences for mothers and infants;3 however, there is a persistent failure to provide intervention for the majority of depressed pregnant women,4 despite the fact that efficacious behavioral interventions exist. A widely recognized barrier to treatment engagement is the lack of effective, available care that is well aligned with women's preferences.5 Pregnant women prefer care that is non-pharmacological6,7 and that is integrated within the obstetric setting,8 and they consult informal sources more than professional ones regarding mental health.9 Thus, to close the gap between treatment need and receipt among depressed pregnant women, it is imperative to examine delivery methods that are efficacious, non-pharmacological, and accessible in the obstetric setting and that expand options beyond traditional professional mental health care. This work is very timely given that recent clinical guidelines require obstetric providers not only to screen for depression but also to initiate medical treatment or refer women who screen positively for depression.10 As a result, many obstetric settings are likely to face increased detection without corresponding availability of mental health services. We address the pressing need for such interventions by developing and pilot testing the model of ?task shifting? to peers, building on work in low- and middle-income countries within the global mental health context.11-18 Behavioral Activation (BA)19-21 is an excellent candidate for task shifting to peers to treat depression during pregnancy because it was developed to maximize scalability, has strong evidence of efficacy in the general population and among pregnant women, a clear and empirically supported conceptual framework, and evidence of efficacy as delivered by non-specialist and lay counselors. Peer delivery offers pragmatic advantages, is consistent with pregnant women's preferences,15 and may engage social putative targets of depression care that are relevant to depression among women.22 Using a three-phase structure, the proposed research will develop BA peer delivery and web-based peer training and fidelity monitoring tools, and will evaluate the feasibility, tolerability, acceptability, safety, and preliminary effectiveness of BA peer delivery within obstetric practice settings. The proposed research also seeks to advance current research paradigms by integrating, within a pragmatic clinical trial context, a conceptually and empirically driven approach to the study of transdiagnostic outcomes and putative targets, consistent with an experimental therapeutics and RDoC approach.23 We combine the use of established self-report measures, which can be routinely used in clinical settings to maximize practice-relevance, and rigorous laboratory paradigms developed to probe key mechanistic processes specific to BA (negative and positive valence system processes) and potentially to peer delivery (social processes).